1. Field of the Invention
The present invention relates to a capacitor charging module, particularly relates to a capacitor charging module utilizing rectifying component to avoid reverse current and negative voltage.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram illustrating a prior art capacitor charging module 100. The voltage on the charged capacitor is always used for a flash module of a camera. The capacitor charging module 100 comprises a power switch 110, a power delivery device 112, a voltage divider 114, a control circuit 116 and a rectifier component 118. The power delivery device 112 is controlled by the power switch 110 for providing a charging current I through the rectifier component 118 to the output capacitor 120 according to a voltage source Vin. The voltage divider 114 is used for providing a feedback voltage VFB according to the voltage on the output capacitor 120. The control circuit 116 is coupled to the voltage divider 114 and the power switch 110, for controlling on/off operation of the power switch 110 according to the feedback voltage VFB. The control circuit 116 turns off the power switch 110 when the voltage level Vout on the output capacitor 120 is at or above the pre-determined value Vref. Conventionally, the power delivery device 112 is a flyback transformer, the power switch 110 is a MOSFET, and the control circuit 116 comprises a comparator 122, a control unit 124, and a driver 126. Also, the voltage divider 114 comprises a resistor 128 and a resistor 130. Since the detail structure and operation of the capacitor charging module 100 is well-known by persons skilled in the art, it is omitted for brevity.
However, there will be a leakage current lleak from the output capacitor 120 to the voltage divider 120 after the output capacitor 120 is charged, and the out voltage Vout will decrease.
FIG. 2 is a schematic diagram illustrating a prior art capacitor charging module 200, which is disclosed in U.S. Pat. No. 6,518,733. The capacitor charging module 200 includes some same devices as a capacitor charging module 100, which comprises a power switch 210, a power delivery device 212, a voltage divider 214, a control circuit 216. The structure of the capacitor charging module 100 and 200 are different, however. As shown in FIG. 2, the structure of the capacitor charging module 200 can prevent the problem of leakage current from the output capacitor 220. According to the capacitor charging module 200, Vout is equal Vref (1+R2/R1)*Ns/Np in this structure, but Ns/Np has tolerance about +−3% due to coupling factor and coil mapping while the fly-back transformer 212 being manufactured. Therefore, the out voltage Vout of FIG. 2 is less accurate than FIG. 1. A more detailed description regarding the circuit is disclosed in U.S. Pat. No. 6,518,733, and is therefore omitted here for brevity.
FIG. 3 is a schematic diagram illustrating a prior art capacitor charging module 300, which is disclosed in US patent 2005/0104560. The capacitor charging module 300 includes the same devices as capacitor charging module 100, which comprises a power switch 310, a power delivery device 312, a voltage divider 314, a control circuit 316. The only difference of the capacitor charging module 100 and 300 is the location of rectifying component 118 and 318. In the capacitor charging module 300, the rectifying diode 318 can prevent the leakage current from the capacitor 330. However, the structure of the capacitor charging module 300 will cause feed back voltage VFB a negative voltage, which is shown as the following equation:VFB=−(Vin*N+Vspike)*R1/(R1+R2), N=Ns/Np 
Where Vspike is a spike voltage happening at the Second winding. The spike voltage is produced by the parasitic capacitance and the secondary leakage inductance of the transformer. A more detailed description regarding the circuit is disclosed in US2005/0104560, and is therefore omitted here for brevity.
FIG. 4 is a schematic diagram illustrating the waveform of the feedback voltage. As shown in FIG. 4, a negative voltage occurs at region A. In general cases, the negative voltage maybe down to −0.8V. An IC (integrated circuit) may be latched up or damaged if the VFB is a negative voltage less than −0.3V, thus causing a serious problem.
Besides, for capacitor charging module 100 and 300, there will be a high voltage from the second winding 113 and 313, thus the diode 118 and 318 suffers a high voltage. As know by persons skilled in the art, an electronic device capable of suffering high voltage is always expensive and has a large area, thereby increases the cost of circuit and decreases the flexibility of design.
Thus, a new invention is needed to solve above-mentioned problems.